Object-holding device for electron microscopes



Oct. 28, 1958 s. LElSEGANG 2,858,444

OBJECT-HOLDING DEVICE FOR ELECTRON MICROSCOPES Filed July 13, 1955 2 Sheets-Sheet 1 Inventor:-

Oct. 28, 1958 s. LEISEGANG OBJECT-HOLDING DEVICE FOR ELECTRON MICROSCOPES Filed July 13, 1955 2 Sheets-Sheet 2 United States Patent p OBJECT-HOLDING DEVICE FOR ELECTRON MICROSCOPES Siegfried Leisegang, Berlin-Zehlendorf, Germany, assignor to Siemens & Halske Aktieugesellschaft, Berlin- Siemensstadt and Munich, Germany, a corporation of Germany Application July 13, 1955, Serial No. 521,852

Claims priority, application. Germany July 14, 1954 13 Claims. (Cl. 250-495) My invention relates to electron microscopes and has for its object to provide for such microscopes an objectholding device that prevents or greatly minimizes contamination of the object and/or affords subjecting an object of organic substance to controlled decomposition by electron irradiation.

In the operation of electron microscopes the occurrence of contamination of the object, being irradiated by electrons, is often encountered as a disturbing and troublesome phenomenon. This is particularly the case in the electron-microscopic irradiation of finest structures. It is known that the contamination is caused by residual gases in the high vacuum, mainly hydrocarbon compounds originating from the oil used in the vacuum pump, the rubber gaskets, or the metal surfaces of the vacuum walls. It is further known that such contamination can be reduced by increasing the temperature of the object and by carefully avoiding the use of pump oil, rubber gaskets and vacuum grease. However, heating the object under examination to a high temperature is not always desirable; and pump oil, rubber gaskets and vacuum grease cannot be completely avoided in such complicated apparatus as electron microscopes.

In electron irradiation apparatus not suitable for microscopic purposes, it has also been attempted experimentally to place the object into a cooling trap, while care was taken to maintain the object itself at normal room temperature or at a higher temperature. However, in electron microscopes, where a relatively high temperature of the object is to be maintained, the provision of a cooling trap encounters great and so far insurmounted difiiculties.

According to the invention, the above-described objects are achieved by subjecting at least a major portion of the chamber walls that limit the half-spaces on both sides respectively of the object, as well as the object itself, to cooling by heat conductance. Experiments have shown that, contrary to the prevailing opinion, the cooling of the object does not result in increased contamination as long as a sufficiently large portion of the surrounding walls is simultaneously cooled.

According to another feature of my invention, the object and the Walls surrounding it are cooled down to a temperature of minus 80 centigrade and below. It has been found that this particular temperature represents a critical point for the effect of the cooling provided according to the present invention. If one approaches this temperature from higher values, then the velocity of contamination decreases down to zero. If the temperature drops below minus 80 centigrade, then the substance of the object begins to decompose, and this decomposition occurs more rapidly with a further reduction in temperature.

2,858,444 Patented Oct. 28, 1958 ing to this embodiment, the environment of the object is cooled virtually on all sides.

According to another feature of the invention, the ob- According to further features of the invention, the chamber is preferably heat-insulated from other parts. of the microscope, and the heat from the chamber is dissipated through a good heat-conducting rod which extends to the outside of the microscope and has its exterior end immersed in a coolant, for instance liquid air. With such a very simple device, sufficiently low temperatures of the cooling chamber can readily be obtained.

According to another feature of the invention, the

cooling chamber is machined into an object-holding cartridge, and the cooling rod is guided through the casing of the objective coil and is pressed by spring force against the object cartridge so that it does not interfere with adjusting movements of the. specimen-supporting table. For measuring the temperature, a thermo-element can be mounted on the object cartridge. The thermo-electrically generated voltage is conducted through the cooling rod to the outside of the microscope so movements of the specimen-supporting table as well as the exchange of the object are not obstructed or impeded. The thermoelement may be used to control a regulating member for maintaining the cooling temperature at a desired constant value.

The favorable effect of the apparatus can be explained physically by the fact that the vapor pressure of the substances causing contamination is extremely slight in the greatly cooled environment of the object. The ingress of vapor molecules into the object space. is very much impeded by the diaphragms at the front and rear of the object. Not fully understood is the phenomenon of decomposition of the object or preparation which has been observed to occur below minus centig'rade. It is assumed, however, that this phenomenon is related to the further diminution of the pressure of the gases and vapors present in the object space.

Further objects and advantages of my invention will become apparent as the following description proceeds; and the features of novelty which characterize my invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

In the drawings:

Fig. 1 shows a sectional view of the object cartridge according to the invention,

Fig. 2 shows a cross-section of a magnetic objective lens of an electron microscope equipped with the new object cartridge,.and

Fig. 3 shows a corresponding cross-section through the microscope tube.

As illustrated in Fig. 1, the object cartridge embodying this invention comprises a generally cylindrical object cartridge member 1 into which is inserted a chamber structure 3, 4 consisting of copper. This structure is heat-insulated from the body of the cartridge 1 by an annular intermediate member 2 of poor heat conductivity, which consists for instance of resinous plastic such as the plastic known under the trademark Trolitul. The interior of the structure 3, 4 forms a cooling chamber 71. The copper bushing 3 is capped at the bottom by screw cap 4. Cap 4 comprises the specimen or object holder. It has a central cavity 40 in which are located the objectholding diaphragm 5 and the closure diaphragm 6. The diaphragms shade off a portion of the electron ray. The object or specimen is indicated at 11 in Fig. l. Bushing 3 is provided with apertures 7 and 70, which are axially aligned with aperture 40 of the cap. These openings form a passage for the electron ray. The cooling chamber 3, 4 is in good heat-conducting connection with a cooling rod 8 of copper whose outer end is immersed in a coolant, as will be apparent from Fig. 2.

Inserted into the bushing 3 of the cooling chamber and firmly joined therewith is a block 9 of a different metal which forms a thermo-element together with the metal of which the bushing 3 is made. Preferably the block 9 consists of a copper-nickel alloy known under the trade-mark Constantan. The voltage of the thermoelement formed by the parts 3 and 9 is etfective between the cooling rod 8 and a centrally located conductive pin 10. The pin has point contact With the block 9 in order to dissipate a minimum amount of heat from the vicinity of the measuring location. Thus it does not disturb the temperature conditions in the vicinity of the thermoelement. This voltage is available at the outside of the microscope for the above-mentioned measuring or automatic regulating purposes as is herein below described.

As illustrated in Fig. 2, the excitation winding of the objective lens is denoted by 110 and the magnetizable enclosure of the lens is denoted by 12. The lens field proper is formed between the two pole shoes 13 and 14, which form the objective lens. The object cartridge is introduced from above into the supporting table 15. The heat conducting rod 8 passes through a bore 16 in the table 15 and is axially displaceably mounted in the outer rim portion 17 of the objective-lens body or casing. A vacuum tight seal is secured by means of a rubber gasket 18. In order to resiliently force the rod 8 against the object cartridge 1, a flange 19 attached to the rod 8 is engaged by a helical pressure spring 20. This spring, together with the flange 19, is held within a heat-insulating sleeve 21. At its exterior end, the rod 8 carries a downwardly-extending copper piece 22 which is immersed in a vessel 23 containing liquefied air for cooling of the rod 8.

In order to maintain the temperature constant under control by the thermo-element 3, 9, the vessel 23 is vertically displaceable and can be raised or lowered by means of a motor 24 through a rack and gear mechanism 25. A contactor 26 controls the motor to operate in one or the other direction. Contactor 26 connects the motor 24 with a current supply line 27. The contactor is in turn controlled by an amplifier 28 connected in the circuit of the thermo-element 3, 9 and which also contains a reference thermo-element 29.

In Fig. 3 the object cartridge or specimen-supporting table 15 and the inserted object cartridge 1 are seen in top view. The devices for displacing the table 15 in coordinate directions and to any desired extent, comprise two control rods 30, 31 which form an angle of 90 with each other. These control rods pass through the body 17 of the objective lens in the conventional, vacuum-tight manner. Two counter bias members 34 and 35 are located opposite the respective control rods 31 and 30 and are guided in the lens body 17 and urged inwardly by respective springs 32 and 34. The cooling rod 8 is located between the counter bias members 34, 35 and is displaced relative to each of them by an angle of 45. The cooling rod 8, as described above, is biased by the spring 20. It will be recognized that it is possible to conduct the. thermo-voltage of the thermo-element 3, 9 through the rod 8 and the central pin to the outside of the microscope housing, without causing the object cartridge to be impeded or obstructed by the crosswise movement of table 15. The rod 8 and pin 10 operate within the range of the object cartridge in cooperation with the sliding surfaces apparent from Fig. 1, so that the table and the cartridge 1 inserted therein can perform their displacing movements without hindrance.

While I have shown and described particular embodiments of my invention, it will occur to those skilled in the art that various changes and modifications may be made Without departing from the invention. I therefore aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

I claim:

1. In an electron microscope, the combination of an object holding device comprising a support, a structure carried by said support and thermally insulated therefrom, said structure forming a chamber having two coaxial apertures which are located on opposite sides respectively of said chamber and are in the electron optical axis of the microscope, said structure having at one axial end thereof an object mounting device comprising a member having a central opening in spaced coaxial disposition with respect to the apertures of said structure, the outer periphery of said member being in heat conductive contact with said structure, and a cooling device having a heat conductive body in heat conductive connection with said structure and extending away from the outer side of said structure to conductively dissipate heat from said chamber and object.

2. In an electron microscope, the combination of an object holding device comprising a support, a structure centrally carried by said support and thermally insulated therefrom, said structure forming a central chamber and having two coaxial apertures which are located on opposite sides respectively of said chamber and are in the electron optical axis of the microscope, said structure having at one axial end thereof a mounting device, a pair of spaced disc members in said mounting device having central openings in spaced coaxial disposition with respect to the apertures of said structure, one of said discs being an object mounting device, the outer periphery of said disc members being in heat conductive contact with said structure, and cooling means in heat conductive contact with said structure for cooling said structure down to at least minus centigrade.

3. In an electron microscope, the combination of an object holding device comprising an annular support, a structure centrally mounted in said support and thermally insulated therefrom, said structure forming a central chamber and having two coaxial apertures which are located on opposite sides respectively of said chamber and are in the electron optical axis of the microscope, a centrally apertured object holding member mounted in said structure, the aperture of said holding member being coaxial with respect to said coaxial apertures, and cooling means for reducing the temperature of said structure and said object holding member down to minus 80 centigrade or lower.

4. In an electron microscope, the combination of an object holding device comprising an annular support, a structure centrally mounted in said support and thermally insulated therefrom, said structure forming a central chamber and having two coaxial apertures which are located on opposite sides respectively of said chamber and are in the electron optical axis of the microscope, a centrally apertured object holding member mounted on said structure, the aperture of said holding member being coaxial with respect to said coaxial apertures, cooling means for reducing the temperature of said structure and said object holding member down to minus 80 centigrade or lower, said cooling means comprising a heat conductive rod having one end in contact with said structure and a heat conductive member immersed at one end in a low temperature coolant, the other end of said heat conductive rod being heat conductively connected with said heat conductive member.

5. In an electron microscope, the combination comprising an annular support member, a structure centrally mounted in said support member and thermally insulated therefrom, said structure forming a central chamber having opposite coaxially spaced end apertures which are in the electron optical axis of the microscope, a centrally apertured object holding member mounted on said structure, the aperture of said holding member being coaxial with respect to said end apertures, an ancured in said table, and cooling means for reducing the temperature of said structure and said object holding member down to minus 80 centigrade' or lower, said cooling means comprising a heat conductive rod having one end'in contact with said structure, a container filled with a low temperature coolant, a heat conductive member partially immersed in said coolant, the other end of said heat conductive rod being joined with said heat conductor rod, and resilient means for constraining said one end of said rod in heat conducting contact with said structure.

6. In an electron microscope, the combination comprising an annular support member, a structure centrally mounted in said support member and thermally insulated therefrom, said structure forming a central chamber having opposite coaxially spaced end apertures which are in the electron optical axis of the microscope, a centrally apertured object holding member mounted on said structure, the aperture of said holding member being coaxial with respect to said end apertures, an annular objective coil member, a displaceable table on said objective coil member, said support member being secured in said table, and cooling means for reducing the temperature of said structure and said object holding member down to minus 80 centigrade or lower, said cooling means comprising a heat conductive rod having one end in contact with said structure, a container filled with a low temperature coolant, a heat conductive member partially immersed at one end in said coolant, the other end of said heat conductive rod being joined with said heat conductor rod, and thermo-electric means associated with said structure for measuring the temperature thereof.

7. In an electron microscope, the combination comprising an annular support member, a structure centrally mounted in said support member and thermally insulated therefrom, said structure forming a central chamber. having opposite coaxially spaced end apertures which are in the electron optical axis of the microscope, a centrally apertured object holding member mounted on saidstructure, the aperture of said holding member being coaxial with respect to said end apertures, an annular objective coil member, a displaceable table on said objective coil member, said support member being secured in said table, and cooling means for reducing the temperature of said structure and said object holding member down to minus 80 centigrade or lower, said cooling means comprising a heat conductive rod having one end in contact with said structure, a container filled with a low temperature coolant, and a heat conductive member partially immersed in said coolant, the other end of said heat conductive rod being joined with said heat conductor rod, thermo-electric means associated with said structure for providing a control voltage proportional to the temperature of said structure, drive mechanism for varying the amount of immersion of said heat conductive member in said coolant, and circuit means for controlling said drive mechanism in accordance with said temperature-proportional voltage for maintaining the temperature of said structure substantially constant.

8. The invention as defined in claim 7 where said circuit means comprises a thermo-element device producing a regulatory reference voltage.

9. In an electron microscope, the combination of an object holding device comprising a support means, a heat conductive structure carried by said support means, the support means and structure being thermally insulated from each other, said structure forming a chamber having two coaxial apertures serving for the passage of electron rays and located on opposite sides respectively of said chamber, an apertured object holding means mounted in said chamber the aperture of which is on the axis defined by said coaxial apertures, and means for chilling the said heat conductive structure comprising an external cooling means located outside the microscope and a heat conductive body in heat conductive connection with thesaid structure and the said cooling means.

10. In an electron microscope, the combination of an object holding device comprising a support means, a heat conductive structure carried by said support means, the support means and structure being thermally insulated from each other, said structure forming a chamber having two coaxial apertures serving for the passage of electron rays and located on opposite sides respectively of said chamber, an apertured object holding means mounted in said chamber the aperture of which is on the axis defined by said coaxial apertures, and means for chilling the said heat conductive structure comprising an external cooling means located outside the microscope and a heat conductive body in heat conductive connection with the said structure and the said cooling means, the object holding means being in heat conducting relation to the structure, whereby it also is chilled.

11. In an electron microscope, the combination of an object holding device comprising a support means, a heat conductive structure carried by said support means, the support means and structure being thermally insulated from each other, said structure forming a chamber having two coaxial apertures serving for the passage of electron rays and located on opposite sides respectively of said chamber, an apertured object holding means mounted in said chamber the aperture of which is on the axis defined by said coaxial apertures, and means for chilling the said heat conductive structure comprising an external cooling means located outside the microscope and a heat conductive body in heat conductive connection with the said structure and the said cooling means, the object holding means being in heat conducting relation to the structure, whereby it also is chilled, the cooling means comprising means capable of cooling the inner walls of the heat conductive structure and the apertured object holding means down to at least below 0 C.

12. In an electron microscope, the combination of an object holding device comprising a support means, a heat conductive structure carried by said support means, the support means and structure being thermally insulated from each other, said structure forming a chamber having two coaxial apertures serving for the passage of electron rays and located on opposite sides respectively of said chamber, an apertured object holding means mounted in said chamber the aperture of which is on the axis defined by said coaxial apertures, and means for chilling the said heat conductive structure comprising an external cooling means located outside the microscope and a heat conductive body in heat conductive connection with the said structure and the said cooling means, and means responsive to the temperature of said heat conductive structure to control the chilling means to control the temperature of said heat conductive structure.

13. In an electron microscope, the combination of an object holding device comprising a support means, a heat conductive structure carried by said support means, the support means and structure being thermally insulated from each other, said structure forming a chamber having two coaxial apertures serving for the passage of electron rays and located on opposite sides respectively of said chamber, an apertured object holding means mounted in said chamber the aperture of which is on the axis defined by said coaxial apertures, and means for chilling the said heat conductive structure comprising an external cooling means located outside the microscope and a heat conductive body in heat conductive connection with the said structure and the said cooling means, the said heat conductive body comprising a hollow rod, an electro-conductive element passing through the hollow rod, a thermo-couple element carried by said heat con- References Cited in the file Of this patent ductive structure in position for contact with said electro- UNITED STATES A S conductive element, the thermo-couple element and the electro-conductive element comprising means responsive 2220973 Manon 1940 5 2,264,210 Krause Nov. 25, 1941 to the temperature of the heat conductive structure to control the chilling means to control the temperature of FOREIGN PATENTS said heat conductive structure. 236,120 Switzerland May 16, 1945 

